Humans are the only host of norovirus, and norovirus has several mechanisms that allow it to spread quickly and easily. Norovirus infects humans in a pathway similar to the influenza virus’ mode of infection. In addition to their similar infective pathways, norovirus and influenza also evolve to avoid the immune system in a similar way. Both viruses are driven by heavy immune selection pressure and antigenic drift, allowing evasion of the immune system, which results in outbreaks. Norovirus can survive a wide range of temperatures and in many different environments. Moreover, the virus can spread quickly, especially in places where people are in close proximity, such as cruise ships and airline flights.
Norovirus causes nearly 60 percent of all foodborne illness outbreaks. It is transmitted primarily through the fecal-oral route, with fewer than 100 norovirus particles needed to cause infection. Transmission occurs either person to person or through contamination of food or water. CDC statistics show that food is the most common vehicle of transmission for noroviruses; of 232 outbreaks between July 1997 and June 2000, 57 percent were foodborne, 16 percent spread from person-to-person, and 3 percent were waterborne. When food is the vehicle of transmission, contamination occurs most often through a food handler improperly handling a food directly before it is eaten.
Infected individuals shed the virus in large numbers in their vomit and stool, shedding the highest number of viral particles while they are ill. Aerosolized vomit has also been implicated as a mode of norovirus transmission. Previously, it was thought that viral shedding ceased approximately 100 hours after infection; however, some individuals continue to shed norovirus long after they have recovered from it, in some cases up to 28 days after experiencing symptoms. Viral shedding can also precede symptoms, which occurs in approximately 30 percent of cases. An infected food handler may not even show symptoms. In these cases, people can carry the same viral load as those who experience symptoms.
Common settings for norovirus outbreaks include restaurants and events with catered meals (36%), nursing homes (23%), schools (13%), and vacation settings or cruise ships (10%). Proper hand washing is the best way to prevent the spread of norovirus.
The good news is that norovirus does not multiply in foods as many bacteria do, and thorough cooking destroys the virus. To avoid norovirus, make sure the food you eat is cooked completely. While traveling in in areas that have polluted water sources, raw vegetables should be washed thoroughly before being served, and travelers should drink only boiled drinks or carbonated bottled beverages without ice.
In 2006, there was a large increase in the number of norovirus cases on cruise ships. Norovirus cases increased throughout Europe and the Pacific at the same time. Close living quarters on cruise ships lead to closer contact between people, and there is still a lack of public understanding regarding how norovirus spreads. The proximity and concentration of the illness on board a ship does lead to faster detection of outbreaks. Cruise ship outbreaks often occur when new strains of norovirus are emerging, making them a helpful indicator system for new norovirus strains.
Outbreaks of norovirus infections have become synonymous with cruise ships. Healthcare facilities also experience a high incidence of outbreaks. The CDC has published information regarding the prevention of norovirus outbreaks on cruise ships and in healthcare facilities on its website. Once a case has occurred, even more stringent hygienic measures are required to prevent an outbreak, particularly in enclosed spaces.